[FAQ]

I’m drowning in subject-line ideas but short on send volume. A full A/B test costs me a chunk of the list every time, so I’m tempted to rely on AI “scorecards” that claim they can predict the winner before I hit send.

Main question: How trustworthy are these AI predictors?

[Jeff Bullas]

Great question—everybody wants to skip the “test tax,” but here’s what the data (and our Vault experiments) show:

1. AI can narrow the field, not guarantee a champion.
Mailchimp’s Subject Line Helper, Phrasee, and similar models tap billions of past emails to flag risky words, ideal length, and tone. They’re good at weeding out obvious duds—our Vault members see about a 15-20 % average lift in open rates when they swap out the red-flag lines the tool identifies. That lines up with Salesforce Marketing Cloud’s 2024 study reporting a 34 % engagement bump after marketers adopted AI-guided subject lines.

2. Real-world outcomes still vary by list and moment.
Nextdoor’s 2025 experiment is a perfect cautionary tale: their first round of AI-generated lines was a wash, only after a prompt tweak did they eke out a +1 % click-through lift—hardly a landslide.

3. Small lists can’t rely on prediction alone.
Prediction models are trained on huge, mixed datasets. If you send to 3,000 subscribers in a niche with its own slang, a “high score” might still flop. For lists under ~10 k, even a micro A/B (10-15 % of the list, 4-hour send window) gives you safer intel than a blind send.

4. Best practice:
Use AI as a filter. Generate or score 5-10 lines, keep the top two.
Run a light A/B. Send each contender to 10 % of your list; winner rolls out to the remaining 80 %.
Iterate the AI prompt. Feed wins and losses back into the model so its next batch is closer to your brand’s voice and audience quirks.

Bottom line
AI is a smart co-pilot—it cuts ideation time and screens out losers—but a quick A/B is still the only way to know what your crowd will click today. Treat prediction as step one, not the finish line, and you’ll get the best of both worlds: speed and certainty.

Hope that saves you a few headaches (and unsubscribes). Keep testing—just faster.

— Jeff

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